Piezoresistive strain concentrator

Abstract

A piezoresistive strain concentrator that converts mechanical movement into electrical output and a process for fabricating the concentrator are provided. The device includes a strain sensing structure composed of a piezoresistive strain sensitive element that spans a gap in a substrate. The strain sensing structure is supported on a strain concentrating structure also spanning the gap that has vertical walls extending to a cross-section at the base of the gap, both structures being etched from the substrate. The structure of the strain-concentrating support for the strain sensitive element is fabricated by deep reactive ion etch (DRIE). The strain sensing structure has an increased sensitivity, a low gage factor and an increased resistance to buckling and fracture compared to previous strain gage structures. Several of the strain sensing structures can be connected in a sequence in a bridge circuit.

1. A device for sensing mechanical input and converting mechanical movement of at least two relatively movable parts into electrical output, comprising:
 a silicon substrate; 
 a gap extending across a portion of the substrate defining the relative moveable parts and a flexible cross-section extending there between, the cross-section derived from the same material as the substrate; 
 at least one strain sensitive element provided on the surface of the silicon substrate, the strain sensitive element having two end portions interconnected by an intermediate neck portion, the neck portion being supported on a structure that concentrates strain, the structure extending across the gap and having vertical walls extending to the cross-section in the gap and the structure derived from the same material as the substrate; and 
 electrode means electrically connected to the end portions for detecting changes in electrical resistance between the end portions, when the neck portion is subjected to stress in the direction of a current through the strain sensitive element that results from the relative movement of the substrate parts. 
 
   
   
     2. The device of  claim 1  wherein the silicon substrate is oriented in the (110) plane and comprises an n-type impurity and the strain sensitive element is aligned in the [111] direction and comprises a p-type impurity. 
   
   
     3. The device of  claim 1  wherein the silicon substrate is oriented in the (100) plane and comprised a p-type impurity and the strain sensitive element is aligned in the [001] direction and comprises an n-type impurity. 
   
   
     4. The device of  claim 1  wherein the cross-section defines an elastic hinge parallel to the substrate. 
   
   
     5. The device of  claim 1  wherein the at least two relatively moveable parts tilt relative to each other about the elastic hinge portion of the cross-section. 
   
   
     6. The device of  claim 1  wherein the strain sensitive element comprises boron lightly doped to a depth of about 2 microns. 
   
   
     7. A device for sensing mechanical input and converting mechanical movement of at least two relatively movable parts into electrical output, comprising: a silicon substrate;
 a gap extending across a portion of the substrate defining the relative moveable parts and a flexible cross-section extending there between, the cross-section derived from the same material as the substrate; 
 at least two strain sensitive elements provided on the surface of the silicon substrate and connected in series, each of the strain sensitive elements having two end portions interconnected by an intermediate neck portion, each neck portion being supported on a corresponding structure that concentrates strain, the structure extending across the gap and having vertical walls extending to the cross-section in the gap and the structures derived from the same material as the substrate; and 
 electrode means electrically connected to the end portions for detecting changes in electrical resistance between the end portions when the neck portions are subjected to stress in the direction of a current through the strain sensitive elements that results from the relative movement of the substrate parts. 
 
   
   
     8. The device of  claim 7  wherein the silicon substrate is oriented in the (110) plane and comprises an n-type impurity and the strain sensitive elements are aligned in the [111] direction and comprise a p-type impurity. 
   
   
     9. The device of  claim 7  wherein the silicon substrate is oriented in the (100) plane and comprised a p-type impurity and the strain sensitive elements are aligned in the [111] direction and comprise an n-type impurity. 
   
   
     10. The device of  claim 7  wherein the cross-section defines an elastic hinge parallel to the substrate. 
   
   
     11. The device of  claim 7  wherein the at least two relatively moveable parts tilt relative to each other about the elastic hinge portion of the cross-section. 
   
   
     12. The device of  claim 7  wherein each strain sensitive element comprises boron lightly doped to a depth of about 2 microns. 
   
   
     13. The device of  claim 7  wherein six strain sensitive elements are provided on the surface of the silicon substrate and connected in series. 
   
   
     14. The device of  claim 13  wherein each strain sensitive element comprises boron heavily doped to a depth of about 0.3 microns. 
   
   
     15. The device of  claim 13  wherein each strain sensitive element has a small width of about 4 microns. 
   
   
     16. A device for sensing mechanical input and converting mechanical movement of at least two relatively movable parts into electrical output, comprising:
 a silicon substrate derived from n-type semi-conductive material oriented in the (110) plane; 
 a triple-bossed diaphragm sculpted on one side of the substrate, the triple-bossed diaphragm comprising a rim, a diaphragm extending across the rim, a central boss and two outer bosses on either side of the central boss and inside of the rim; 
 four gaps extending across a portion of the substrate defining the relative moveable parts and flexible cross-sections extending between corresponding gaps such that two of the gaps are located outside the outer bosses and two of the gaps are located on either side of the central boss and inside of either outer boss; 
 four strain sensitive element pairs provided on the surface of the silicon substrate around the gaps, each pair comprising two strain sensitive elements connected in a series, each strain sensitive element having two end portions interconnected by an intermediate neck portion, the neck portion being supported on a corresponding structure that concentrates strain, the structure extending across the gap and having vertical walls extending to the cross-section in the corresponding gap, the strain sensitive elements derived from p-type semi-conductive material and oriented in the [111] direction, the four strain sensitive element pairs connected as a bridge circuit; and 
 electrode means electrically connected to the end portions of the strain sensitive elements for detecting changes in electrical resistance between the end portions, when the neck portions are subjected to stress in the direction of a current through the strain sensitive elements that results from the relative movement of the substrate parts. 
 
   
   
     17. The device of  claim 16  wherein the cross-section defines an elastic hinge parallel to the substrate. 
   
   
     18. The device of  claim 16  wherein the at least two relatively moveable parts tilt relative to each other about the elastic hinge portion of the cross-section. 
   
   
     19. The device of  claim 16  further comprising an insulated crossover at the gage level on the substrate such that the strain sensitive elements are connected to terminals at the corners of the rim in a bridge circuit sequence such that adjacent legs of the bridge have opposite senses of strain. 
   
   
     20. The device of  claim 16  further comprising a torsion bar conduction path from the tilting boss to the terminals on the rim. 
   
   
     21. The device of  claim 16  further comprising a reference cavity in the terminal block sealed by thermocompression bonds to capture a reference pressure. 
   
   
     22. The device of  claim 16  wherein the strain sensitive elements are sealed to a surface mount terminal by thermocompression bonds. 
   
   
     23. The device of  claim 16  wherein each strain sensitive element comprises boron doped to a level of approximately 3×10 18  per cubic centimeter. 
   
   
     24. The device of  claim 16  adapted for deposit on a catheter to measure fluid pressure.